1. Field of the Invention
The present invention relates to a charging device for uniformly charging a surface (subject surface) of a body to be charged (subject body) used in an electrophotographic apparatus or the like to a given potential, and more particularly to a contact charging device for charging a subject surface in such a condition that a charging member is in contact with the subject surface.
2. Description of the Prior Art
FIG. 4 shows an exemplary printing section 10 forming a part of an electrophotographic apparatus, copying machine, facsimile machine, etc.
Referring to FIG. 4, reference numeral 1 denotes an image supporting body (subject body) formed as a cylindrical body whose outer circumferential surface is coated with a photosensitive material. Provided around the image supporting body 1 are a contact charging device 20, exposure means 2, developing roller 3 constituting a developing unit, transfer unit 4, waste toner blade 5, and erasing unit 6.
The image supporting body 1 which is completely round is adapted to rotate about its center Q in a direction of arrow R at a constant speed. A subject surface 1a is formed on the outer circumferential surface of the image supporting body 1. The subject surface 1a is uniformly charged to a given potential Vc (e.g., -500 V) when passing a position opposed to the contact charging device 20. Then, the subject surface 1a is exposed to light directed from the exposure means 2 to form an exposed portion (electrostatic latent image) charged to a given potential Vim (e.g., -50 V). The electrostatic latent image formed on the subject surface 1a is supplied with a toner T from the developing roller 3 to form a toner image as a developed image.
The toner image formed on the subject surface 1a is transferred onto a sheet of paper P moving in a direction of arrow X when passing a position opposed to the transfer unit 4. Thereafter, the toner image transferred onto the paper P is moved to a fuser (not shown). On the other hand, the subject surface 1a having passed the position opposed to the transfer unit 4 comes to the waste toner blade 5, by which a remaining toner is removed from the subject surface 1a. Then, the charge on the subject surface 1a is uniformly erased to an initial potential (e.g., -50 V) by the erasing unit 6.
Conventionally, the contact charging device 20 shown in FIG. 4 and a corona discharge type charging device are selectively adopted as a charging device. The corona discharge type charging device has various defects such as an increase in cost due to provision of a very high-voltage power unit, an increase in space due to formation of a shield chamber, the generation of ozone due to corona discharge, and defective charging due to deterioration of a discharging wire. Therefore, the contact charging device is generally adopted in many cases.
The contact charging device 20 shown in FIG. 4 is composed of a charging roller 101 forming a charging member and a very high-voltage power unit 21 for applying a very high voltage (e.g., -5 kV) to the charging roller 101. The charging roller 101 is composed of a round shaft member 29 and a conductive portion 28 attached to the outer circumferential surface of the round shaft member 29. The conductive portion 28 is formed of rubber in which conductive particles are mixed. In general, the charging roller 101 is adapted to rotate as following the rotation of the image supporting body 1.
In operating the printing section 10, the conductive portion 28 of the charging roller 101 is brought into contact with the subject surface 1a formed on the outer circumferential surface of the image supporting body 1 which surface 1a is moved in the direction R at a constant speed, thereby charging the subject surface 1a to a given potential (e.g., -500 V).
The problems of the prior art as mentioned above will now be described. In the contact charging device as shown in FIG. 4, only when all conditions for uniform charging of the subject surface 1a are satisfied, the subject surface 1a can be uniformly charged. However, because increasing high-quality printing has been strongly demanded in recent years, such all conditions are difficult to satisfy in an actual charging device. Accordingly, it is difficult to uniformly charge the subject surface 1a. For instance, if the uniformity of the composition material of the conductive portion 28 is lacking for such a reason that there are variations in distribution of the conductive particles, the subject surface 1a cannot be uniformly charged. Also, if the outer diameter of the image supporting body 1 is not uniform in the axial direction of the image supporting body 1, the subject surface 1a cannot be uniformly charged.
Furthermore, in the contact charging device 20 as shown in FIG. 4, it is difficult to mount the device 20 so that a contact pressure of the conductive portion 28 against the subject surface 1a falls within a given range, and it is also difficult to stably maintain the pressure contact condition of the conductive portion 28 during the operation. If the contact pressure of the conductive portion 28 against the subject surface 1a is too small, the nonuniformity of charging occurs because of an increase in uncharged portion of the subject surface 1a. Conversely, if the contact pressure is too large, direct charge injection occurs in addition to an original discharging operation, causing an increase in charge on the subject surface 1a and simultaneously causing damage to the subject surface 1a and the conductive portion 28. As a result, high-quality printing cannot be ensured.
In addition, the charge on the subject surface 1a varies with a surface moving speed of the conductive portion 28. If the surface moving speed of the conductive portion 28 is low, the time of contact between the image supporting body 1 and the conductive portion 28 is very long, causing the acceleration of occurrence of the charge injection mentioned above. In this case, the charge potential (Vim) of the subject surface 1a is increased to cause the generation of white lines on a printed surface. This is due to the fact that reversal development is performed in such a manner as not to deposit a toner on a high-potential portion of the subject surface 1a but to deposit the toner on a low-potential portion of the subject surface 1a formed by the exposure means. This high-potential portion on which no toner is deposited causes the generation of white lines on a printed surface. Conversely, if the surface moving speed of the conductive portion 28 is high, a motor capacity needs to be large to cause an increase in power consumption, which is inconvenient from a viewpoint of structure of the device.